1. Field of the Invention
The present invention relates to a pump of the positive-displacement type and, more particularly, to an improvement in such a pump which utilizes a longitudinally-contractable bellows as the displacement means to pump relatively viscous fluids.
2. Description of the Prior Art
Positive displacement pumps utilizing bellows are well known. In such pumps, the fluid to be pumped enters a hollow tubular bellows through a one-way check valve. Usually, the discharge end of the bellows is constrained from movement, while the other end is connected to a reciprocating means which selectively works the bellows for longitudinal expansion and contraction. When contracted, fluid is expelled from the bellows under pressure, which is to say, pumped. It is a well known characteristic of such pumps that, at high pumping pressures, considerable internal pressure is exerted on the bellows which, together with flexing during expansion and contraction, often results in fatigue and early rupture of the bellows.
One application of bellows-type displacement pumps is the metered pumping of fluids. Bellows pumps are well suited for such usage because, each time the bellows is filled and then compressed, a relatively equal quantity of pumped fluid is discharged. Such a consistent metering capability is highly desirable in situations where the objective is to consistently dispense relatively precise quantities of fluid. An example of one such dispensing application is in the semi-conductor industry where the pumped fluid may be highly viscous photoresist which is applied to silicon wafers during the manufacture of integrated circuits. In such metering applications it can be appreciated that, after dispensing, drips of the pumped fluid are to be avoided. To alleviate dripping, auxillary devices known as "drawbacks" have been utilized. Typically, such devices create a negative pressure in the discharge line immediately after the pump discharge stroke is completed, and this keeps residual droplets from being discharged from the metering system.